1. Field of the Invention
The present invention relates to a ceramic heater, and more particularly to a ceramic heater for heating an oxygen sensor used with an automobile, for use in a glow system of a diesel engine, for heating a semiconductor substrate, for use in a fan heater, or the like.
2. Prior Art
The above-mentioned ceramic heater is known to have a structure in which a resistance heating element formed from a metal having a high melting point such as W (tungsten) is embedded in a ceramic substrate formed into a flat shape, a cylindrical shape, or other shape. Such a ceramic heater is manufactured, for example, by the steps of: forming an unfired ceramic compact through sheet forming, extrusion, or a like process; forming a heating element pattern on the ceramic compact through use of paste which contains a high-melting-point metal powder and through thick-film printing or a like method; placing another ceramic compact thereon to obtain a layered assembly; and firing the assembly.
Conventionally, when a ceramic heater of this kind has been used continuously at a high temperature over a long period of time, the resistance heating element has tended to deteriorate and suffer an increase in electric resistance, causing a shortening of service life of the heater. Such a deterioration in the resistance heating element is said to be caused by an electrochemical diffusion phenomenon, so-called electromigration (hereinafter, referred to merely as migration), in which a component of the resistance heating element or a component of the ceramic substrate electrochemically diffuses due to application of current for establishment of a high temperature (for example, in Japanese Patent Application Laid-Open No. 4-329291). For example, when a component of the resistance heating element diffuses into the ceramic substrate through migration, the resistance heating element is consumed at a portion from which the component has diffused out, and may suffer an excessive temperature rise or a disconnection. A metal oxide component, such as MgO or CaO, added as a sintering aid component is present in the form of glass phase within the ceramic substrate. Metal ions or oxygen ions contained in the glass phase also tend to migrate. For example, when the main component of the resistance heating element is W, the resistance heating element is oxidized by migrating oxygen ions and may suffer an increase in resistance, a disconnection, or a like problem.